Camptothecin is an alkaloid extracted from Camptotheca acuminata (Nyssaceae), first described by Wall and Wani in 1966 (J. Am. Chem. Soc. 1966, 88, 3888-3890). Camptothecin, albeit endowed with wide spectrum antitumor activity, especially against colon tumor and other solid tumors and leukemias, is not used in therapy due to its high toxicity, which is particularly manifested in the form of hemorrhagic cystitis, gastrointestinal toxicity and myelosuppression.
A number of camptothecin analogues have been synthesized in order to obtain compounds having low toxicity and high solubility. At present, two drugs are used in clinical practice, namely CPT-11 and topotecan. Other derivatives, such as belotecan, rubitecan, exatecan, gimatecan, pegamotecan, lurtotecan, karenitecin, afeletecan, homocamptothecin, diflomotecan, and many others, are undergoing clinical experimentation. Compound CPT-11 is a highly soluble pro-drug for 10-hydroxy-7-ethylcamptothecin (commonly known as SN-38), approved for the treatment of many solid tumors and ascites (colorectal, skin, stomach, lung, cervice, ovary, non-Hodgkin lymphoma).
Topotecan is a compound soluble in physiological solution, active against the tumors of the lung, stomach, liver, ovary, breast, prostate, esophagus, rectum, soft tissues sarcomas, head and neck, glioblastoma, chronic and acute myelocytic leukemias. Topotecan shows, however, important side effects such as neutropenia and thrombocytopenia.
Lurtotecan is a more soluble derivative, having activity in tumors of the neck, ovary, breast, colo-rectal, and pulmonary microcytoma. However, Lurtotecan also has hematic toxicity.
Rubitecan is a prodrug for the oral use effective against tumors of the pancreas, ovary and breast.
Camptothecin and its analogues, as is the case with all topoisomerase I inhibitors, are effective against tumors resistant to conventional drugs, including topoisomerase II inhibitors; maintain high topoisomerase levels during the whole cell cycle; do not induce multi-drug resistance (Pgo or MRP) or detoxifying metabolism mediated by the enzyme.
Research is now focused on novel inhibitors of the topoisomerase I having lower toxicity than the presently used drugs.
Open-ring camptothecin derivatives show high protein binding (in particular with albumin) and low distribution in the tumor tissues. As a consequence, the product accumulates in the body and tumors are poorly affected.
Conversely, the high lipophilicity of the lactone form promotes the adhesion of camptothecin derivatives to cell membranes, particularly erythrocytes, affecting the tissue/plasma distribution ratio. For this reason, research is being focused towards two alternative approaches: a) design of low protein binding products still having good solubility; b) design of highly potent products having therapeutical effect even at extremely low doses.
Modifications at the 7-, 9-, 10- and 11-positions usually proved well tolerated while not affecting the stability of the DNA-Topoisomerase I-camptothecin ternary complex, the formation of which is responsible for the antitumor activity of the compounds.
Products with 20R configuration proved either inactive or very less active than the products with 20S configuration—which coincides with the natural configuration.
As a rule, modifications at the 5-position are considered unfavourable to the formation of the ternary complex, whereas modifications at the pyridone rings D and E have bee reported to be deleterious to the activity of the product.